(1) Field of the Invention
The present invention relates in general to the shape melting welding process for constructing components, and, in particular, to a new and novel method and apparatus for eliminating microfissure defects in shape melted austenitic material components by closely controlling the deposited weld bead shape to a specific geometry.
(2) Description of the Related Art
As used herein, the term shape melting is used to refer to a welding process whereby structural components are manufactured entirely from weld material deposited layer-upon-layer onto a surface or an initial preform until the desired geometry is achieved. This process offers the advantage of delivering a near net shape component that can be produced quickly and economically using either ferrous or non-ferrous materials.
Automation and computer control can be made integral parts of a shape melting process, thus allowing for maximum flexibility of the shape melting production station. In the initial stages of a project, the design criteria for a particular component to be created by the shape melting process would be subjected to a computerized analysis. Assuming the flexibility to meet the design criteria existed within the shape melting production station, the desired welding parameters could then be programmed into the equipment and functions which control the process.
Since the shape melting process creates components made entirely out of deposited weld metal, the application of automated computer control to the process can result in a final product with "custom tailored" mechanical, corrosion and physical properties. This comes about because these properties are strongly tied to the interrelated functions of weld heat input, cooling rates, bead size and shape, bead sequence and bead position. If, in addition to the above variables, one also permits the composition of the weld filler material to vary in a controlled manner throughout the product, the component can have a desired combination of strength, toughness, hardness or corrosion resistance at critical points.
Special concerns exist, however, when austenitic materials are to be used to produce components by the shape melting process. As used herein, the term austenitic material is intended to refer to the weldable grades of austenitic stainless steels, nickel based superalloys, and iron-nickel based alloys that are particularly weldable by common arc-welding processes such as gas metal arc welding (GMAW), submerged arc welding (SAW), or plasma gas metal arc welding (P-GMAW). One example of such an austenitic material is Inconel 625, a trademark of the International Nickel Company.
The austenitic materials are valued for their unique combination of strength and corrosion resistance. However, when thick and/or highly restrained weld deposits are built up using austenitic materials, a condition known as microfissuring can be engendered. The occurrence of microfissuring is extremely important since it directly impacts the strength of the weld, or, in the case of a shape melted component made from austenitic materials, affects the soundness of the entire component. For the case of shape melting then, these austenitic materials are generally deemed "sensitive materials", since thick weld build-ups of complex shapes are being manufactured and high levels of restraint are anticipated. The fact that these materials are "sensitive", however, does not eliminate the need or desirability of shape melted components made from austenitic materials.
Various investigators have developed methods and apparatus related to the shape melting process. Brandi, deceased, et al (U.S. Pat. No. 3,985,995) discloses a method of making large, structural, one-piece shafts, such as those used in turbines and electric generators, made entirely from weld metal. Pre and post-cooling is used around the point of weld metal deposition to locally cool the workpiece below the martensitic temperature to obtain a bainite or ferrite/pearlite crystalline structure. The method is indicated as being applicable to low carbon steel, unalloyed steel, and low alloy steel (up to 5% Cr and/or 5% Ni).
Million, et al (U.S. Pat. No. 4,517,434) discloses a method and apparatus for manufacturing a tube bend, such as an elbow, by the shape melting process. A base weld ring of the required diameter of the bend is applied to a plate which is capable of downward rotation away from the welding heads, along the radius of curvature desired in the tube bend, and to the degree of the bend desired (30, 45, 90 degrees, etc) as successive layers of weld material are applied. Plural welding heads can be used to make the outer wall of the bend section of a ferritic material, while an inside plating of an austenitic material is being sequentially applied.
Ruckdeschel (Federal Republic of Germany Public Disclosure No. 24 45 891--Public Disclosure Date Apr. 8, 1976) discloses a process and device for applying a surface coating to a cylindrical object using strip or wire electrodes and a plurality of welding heads. The cylindrical object is continuously rotated during the process while the strip electrodes are helically melted thereon, and results in an increase in the speed of the process. The application of the weld beads leaves gaps that are filled in when the successive layers are applied. If wider surface-welded beads are to be created with wire electrodes, the individual wire heads can be moved in an oscillating manner. Other than the resulting wider beads, however, the particular relationship between such oscillation and the resulting overall bead shape, or the benefits of a particular bead shape to the properties of the component being produced, is not disclosed.
Million, et al (European Patent Office Publication No. 0,163,828 A1--Public Disclosure Date Dec. 11, 1985) discloses a process for manufacturing a structural component from weld material using multiple-layer surface welding. During the process, the structure of a weld bead layer is transformed once or several times during the application of subsequent layers in which the weld-bead shape, melting depth, penetration depth and depth of the underlying coarse-grained or fine-grained transformed structural zones are entirely transformed into a zone of fine-grained structure. The process is characterized by the fact that it is particularly well suited to producing weld material whose material qualities correspond to the 10MnMoNi 5 5 grade of steel. The 10MnMoNi 5 5 grade of steel is similar to ASTM A533 Gr. B, Cl. 1 or ASTM A508 Cl. 3, which are ferritic steels. To control the desired thermal transformation of the heat affected zone, a weld bead geometry corresponding to a flat weld bead with a more lens-shaped cross-section is used. It is to be noted that there are no particular teachings in EPO Publication No. 0,163,828 A1 as to how the particular weld bead geometry is to be attained. Further, the particular goal of transforming the heat affected zone of the underlying layers to obtain a fine-grained structure indicates that the disclosure is primarily directed to ferritic materials (that do not suffer from the above-described microfissuring problems), since such a grain structure is not achieved with the above-mentioned austenitic materials by such a thermal process.
It has thus become desirable to develop an improved method and apparatus for use in the production of shape melted components made from austenitic materials that can eliminate the potential for microfissuring type defects in the components themselves.